Known cellular phenomena related to acidic organelles include endocytosis, exocytosis, and autophagy, and acidic pHs within intracellular vesicles during these processes are known to affect vesicular membrane transport and hydrolase activity within the vesicular lumina, and, by extension, playing a major role in cellular functioning. The development of techniques of measuring intracellular vesicular pH is desirable in order to understand these phenomena, and pH probes that alter fluorescence properties in acidic environments promise to be useful tools.
The inventors have previously developed various pH probes using a BODIPY fluorophore (Urano, Y. et al., Nat. Med. 15, 104, 2009; WO/2008/059910). However, performing high-magnification observation of the microscopic structures of intracellular vesicles requires high-intensity excitation light, which causes these pH probes to fade, impeding pH measurement.
As such, there are as of yet no reports of a photostable pH probe that only exhibits strong fluorescence within intracellular acidic environments and is capable of withstanding fluorescent imaging of the microscopic structures of intracellular vesicles.